1) Field of the Invention
The present invention relates to a field-sequential display device and a method of color displaying using the display device.
2) Description of the Related Art
One of the popular methods of multicolor displaying in a field-sequential display device is to divide a field into several subfields, emit a light of a specific color within a part of a time period of the subfield, and at the same time, display an image that corresponds to the light on an displaying unit, by configuring a display device with a light source that emits a plurality of color lights, each of which can be controlled independently and the displaying unit that controls either of transmission or reflection of the light from the light source and reflection of an external light.
In order to realize the field-sequential display device that can display multiple colors, three color (RGB) light sources with a high speed switching capability is necessary. In the past, since an optimal light source was not available, the field-sequential device was only employed to display specific colors, such as a simple guide plate based on about four colors. However, rapid improvement of blue LEDs and high luminance of green LEDs enabled colors of red, blue and green to be obtained with high luminance, and now the three colors can be used as the light sources of the field-sequential display for displaying full color images with high performance.
Since the red, blue, and green LEDs have a broader color reproduction range on a chromaticity diagram than a color filter display device, colors not conventionally available can now be represented, thereby it is possible to display more faithful and beautiful images. Furthermore, since a color filter is not used, it is possible to obtain a high transmittance and a low electrical power consumption of backlight, resulting in an energy saving effect of a whole system. From these advantages, development of the field-sequential display device is being rapidly advanced (for example, see Japanese Patent Application Laid-Open Publication No. 11-52354 (1999)).
FIG. 10 illustrates display timing of a conventional display device. In the display device, an LED is used as a light emitting element, and a liquid crystal panel is used in a displaying unit. An area “a” indicates light emitting timing of each color in the backlight LEDs arranged on a rear surface of the liquid crystal panel, and an area “b” indicates scanning timing and displaying time of each line on the liquid crystal panel.
In the example shown in FIG. 10, in order to obtain color displaying using an integration effect in the time axis direction of a human eye, a field frequency (“field” shown in FIG. 10) is set to 100 Hz. One field is divided into three subfields and comprises an R subfield fr for turning a red LED on, a G subfield fg for turning a green LED on, and a B subfield fb for turning a blue LED on. As shown in the area “a”, each LED of the color corresponding to each subfield emits a light for a fixed emitting time Tb in the latter part of each subfield.
Each subfield of the liquid crystal panel comprises a writing time Tw, a responding time Tr, and a displaying time Td. During the writing time Tw, an electric voltage is supplied based on pixel data while scanning each pixel of the liquid crystal panel sequentially, and transmittance is adjusted. The responding time Tr, which is set to be shorter than the writing time Tw, is from the end of the writing time Tw until obtaining of a desired image on a full screen based on a response of the liquid crystal. The rest is the displaying time Td for which the desired image is displayed.
In the area “a”, the light emitting time Tb is set in such a manner that the displaying times are equal, and the LED is turned on only for the displaying time Td. This produces an effect that a color mixing is prevented by allowing the LED to emit only for a time for which the image displaying is defined. If the LED starts to emit the light, for example, during the writing time Tw, an image of a previous subfield remains on a portion where the scanning of each line is not ended or a portion where the liquid crystal does not respond. This results in a time for which the image does not match with the luminescent colors, and this may cause the color mixing.
As described above, the conventional technology emits the LEDs of each color in the backlight sequentially in order of red, green and blue and displays images on the liquid crystal panel corresponding to each color light in synchronization with the light emitting to realize a color display. Furthermore, by using a liquid crystal panel with a capability of displaying multi-gradation, it is possible to realize a display in full-color.
When comparing the color filter type display device with the field-sequential display device, the transmittance of the liquid crystal display device shows a great difference. Since the liquid crystal panel of the field-sequential display device is a simple monochrome one, the transmittance is higher than 35%, while the transmittance of the liquid crystal panel into which a color filter is incorporated is about 10%.
Therefore, even when both devices are used as transmission-type display devices using the backlight, the field-sequential display device enables color displaying with higher brightness in comparison with the color filter display device. When both devices are used as reflection-type display devices using an intense external light, the color filter display device cannot display an image because of a contrast. On the contrary, the field-sequential display device has a merit that a sufficient displaying is possible, and thus it is suggested to use the field-sequential display device both as the transmission-type display device and the reflection-type display device (for example, see Japanese Patent Application Laid-Open Publication No. 2002-203411).
FIG. 11 illustrates a problem occurring when the conventional display device used in a cellular terminal. The cellular terminal 1200 is frequently used in an environment where the external light is bright such as the outdoors, and thus the display device should be visually recognized satisfactorily regardless of the indoors and the outdoors.
In the indoors where the light intensity is relatively low, a sufficient visibility can be obtained as the transmission-type display device by the backlight, however, since the sunlight 1205 with an intensity of nearly 100 times higher than that in the indoors enters a liquid crystal screen 1201 in the outdoors, the visibility in the outdoors becomes greatly lower than the visibility in the indoors. As a countermeasure against this problem, the cellular terminal 1200 can be covered by one hand so that the sunlight 1205 is blocked. However, since the sunlight 1205 is actually a scattered light, the intensity of incident light is not expected to be reduced remarkably, and thus the sufficient visibility cannot be obtained as the transmission-type display device.
With reference to FIG. 12, a reflection-type displaying operation of the field-sequential display device is explained below. When the sunlight 1205 enters the liquid crystal screen 1201, the light is reflected due to a difference in refractive index on an interface between a windscreen 1202 arranged on the liquid crystal screen 1201 and an air layer, and on an interface between a surface of the liquid crystal screen 1201 and the air layer. Before entering the liquid crystal screen 1201, reflected light 1207 that is about 10% of the sunlight 1205 reaches a user.
Since the color filter is not used, the transmittance of the liquid crystal screen 1201 is about 35%. Therefore, 35% of the sunlight in 90% of the sunlight entering the liquid crystal screen 1201 enters and is reflected by the backlight 1203 so as to again enter the liquid crystal screen 1201. If polarized light is not eliminated at this time, the sunlight is not absorbed by the color filter, and thus 100% of the sunlight transmits directly.
The intensity of reflected light 1211 returning to the visible side, therefore, becomes about 32% of the sunlight 1205. The contrast, thereby, becomes as follows:Contrast=(L×42%)/(L×10%)=4.3This value is about four times as large as that of the color filter display device. When the contrast is 4.3, not only characters but also images can be sufficiently recognized. Brightness of white displaying (L×42%) becomes three times as high as that of the color filter display device, thereby enabling displaying with good visibility. In the field-sequential display device, acceptable reflection-type displaying using the external light, which is impossible in the color filter display device, becomes possible, and thus the field-sequential display device can be used both as the transmission-type display device and the reflection-type display device that can obtain the acceptable visibility in both the indoors and the outdoors.
However, since the conventional technology works basically under a condition that the transmission displaying unit whose light source is the backlight is used, the following problem arises.
In the field-sequential display device according to the conventional technology, as shown in FIG. 5 of Japanese Patent Application Laid-Open No. 11-52354 (1999) and FIG. 6 of Japanese Patent Application Laid-Open No. 2002-203411, the three subfields of R, G and B are obtained by dividing one field into three of the same duration. Transmission-type displaying and reflection-type displaying operations in the field-sequential display device having the subfields of the same duration are explained below with reference to FIG. 13 and FIG. 14. FIG. 13 and FIG. 14 illustrate examples of a color bar displaying, rather than the image displaying, in order to clarify the difference between the transmission-type displaying and the reflection-type displaying.
FIG. 13 is a pattern diagram of display states in various photo-environments. Arrows shown in FIG. 13 relatively indicates the photo-environments: the arrow 13 represents the external light, 0 means that the intensity of light is zero in a dark room or the like, and 100 shows that the intensity of the light is a maximum in the outdoors under fine weather. In the indoors such as a normal office, the intensity of light corresponds to about 30.
On the other hand, the arrow 14 represents the backlight intensity. The backlight intensity is always 10 because it is constant regardless of environments. The bottom left of FIG. 13 illustrates the display state in which the intensity of the external light is zero at the time of displaying the color bars using the field-sequential display device. When the intensity of the external light is zero, a reflected component of the external light does not exist, and thus the emitted light of the color light by means of the field-sequential driving is visually recognized directly as the transmission-type displaying, so that the color bars are displayed with high color saturation.
The bottom right of FIG. 13 illustrates the display state in which the intensity of the external light is 100 corresponding to the outdoors under fine weather. When the external light is stronger than the intensity 10 of the backlight, the transmission-type color displaying using the backlight is hardly recognized visually, and thus the reflection-type displaying using the external light is dominant.
A black color that is displayed on the left end of the color bar displaying in FIG. 13 is visually recognized directly as black. On a blue display section, the transmission-type displaying is obtained only in the subfield fb of FIG. 10, and non-transmission-type displaying is obtained in the other subfields fr and fg. The external light, therefore, reflects only in the subfield fb and does not reflect in the subfields fr and fg. This state for each color is shown in FIG. 14.
The transmission and non-transmission of the liquid crystal panel in each subfield are shown in FIG. 14 by white and black squares. The display color section 17 corresponds to the color bar displaying of FIG. 13 and illustrates the transmission-type display colors by means of the backlight when the intensity of the external light is zero. A gradation display section 18 shows a ratio that black (non-transmission) appears in the three subfields with respect to the respective display colors. This is repeated in the respective fields, and when a human eye recognize that sufficient integration is made during one field, a number of non-transmission appearances can be visually recognized directly as the gradation displaying. That is to say, four-gradation displaying of 0/3, 1/3, 2/3 and 3/3 in the three subfields are executed.
When the intensity of the external light is 100 and thus brighter than the backlight, the reflection monochrome displaying using the external light is visually recognized by the human eye, and as shown in the gradation display section 18, the three colors of blue, red, and green are recognized as the monochrome gradation displaying of 1/3, and three colors of magenta, cyan, and yellow are recognized as the monochrome gradation displaying of 2/3. For example, the color bars are displayed, six kinds of the color displaying from blue to yellow in the bottom left of FIG. 13 becomes only two-gradation displaying including 2/3 gradation displaying and 1/3 gradation displaying in the bottom right of FIG. 13. Six kinds of color displaying contents in the color displaying of the transmission-type displaying are, therefore, displayed with only two gradations in the reflection-type displaying, thereby arising a problem that the contents of the color bars cannot be discriminated.
Even in the case of character displaying or the like other than the color bar displaying, if red characters are displayed on a blue background, for example, when the external light becomes gradually intense and the reflected component is increased, the blue and the red, therefore, become nearly 2/3 gradation displaying, as shown in FIG. 14, and as the external light becomes more intense, it is gradually difficult to discriminate these colors, and then they cannot be discriminated at all. This is applied also to combinations of other colors, and thus the colors that obtain the same gradation in the gradation display section 18 shown in FIG. 14 cannot be discriminated.
When the display device is used in an environment of an intermediate state where the intensity of the external light changes from 0 to 100, the color becomes unnatural. In the field-sequential displaying where the reflection of the external light is taken into consideration, it is natural that the color displaying by means of the backlight is considered to be corresponding to a color adjuster of a television device. That is to say, when the external light is intense, the color displaying by means of the backlight corresponds to a state that the color adjuster narrows down the color.
When the intensity of the external light is 100, the color becomes zero (the backlight becomes invisible) and the transmission-type color displaying shown in the bottom left of FIG. 13 is impossible, but the color bar displaying is replaced by the monochrome bar displaying shown in the bottom right of FIG. 13. It is natural that the monochrome bar displaying becomes monochrome displaying with eight gradations including from black to white of 7/7, 617, 5/7, . . . 1/7 and 0/7 in order of visibility. For example, when green is compared with magenta, if the intensity of the external light is 100, green should be brighter than magenta. As shown in the gradation display section 18 in the bottom right of FIG. 13 and FIG. 14, however, green is 2/3 gradation displaying and magenta is 1/3 gradation displaying, and thus green is darker than the magenta.
That is to say, when the external light changes in the display state, a color component of the reflection-type displaying using the external light is superposed on a color component of the transmission-type displaying using the backlight. The brightness/darkness of green and magenta is inverted in result, the displaying of dark green and bright magenta is obtained, and this is unnatural as the color bar displaying from the viewpoint of the visibility. This is a problem that arises because luminance components and color components of the respective color displaying do not match with each other.
In the conventional technology, as described above, when the reflection-type displaying is executed in an environment that the external light is intense, a display image cannot be recognized with a specific color, and since the color components and the luminance components of the colors do not match with each other, the transmission-type displaying and the reflection-type displaying are brought into an unnatural display state from the view point of the visibility.